33 Concurrency support library [thread]

33.8 Semaphore [thread.sema]

33.8.1 General [thread.sema.general]

Semaphores are lightweight synchronization primitives used to constrain concurrent access to a shared resource.
They are widely used to implement other synchronization primitives and, whenever both are applicable, can be more efficient than condition variables.
A counting semaphore is a semaphore object that models a non-negative resource count.
A binary semaphore is a semaphore object that has only two states.
A binary semaphore should be more efficient than the default implementation of a counting semaphore with a unit resource count.